The present day method of forming laminated curvilinear glass sheets, such as automotive windshields, is dependent upon the simultaneous formation of a pair of matched layers which are subsequently laminated together into a composite laminated curvilinear glass sheet. Not only are the layers simultaneously sagged together by means of a peripheral mold, but also the composition of the various layers must be virtually identical so as to obtain uniform sagging characteristics.
In view of the fact that the peripheral molds utilized in the simultaneous sagging of the two layers only provide for uniform edge contours which remain consistent with the consecutive sagging of various layers, the freely sagged central portion may vary between supposedly identical sagging cycles due to inherent thermal discontinuities within the heat-sagging cycle and variations in mold movement. Thus, although the simultaneously sagged layers, being subjected to identical thermal conditions, will sag into a complementary relation so as to facilitate subsequent lamination into a composite laminate, due to the variations in thermal application between individual cycles, it is virtually impossible to effectively laminate a layer from one free sagging cycle with the layer from another such cycle due to discontinuities in the resulting freely-sagged proportions of such layers. Accordingly, after a pair of layers have been simultaneously sagged by means of a peripheral mold, such layers must be maintained as "partners" until such time as they are subsequently "married" into a composite laminate. Should one such layer of a simultaneously free-sagged pair of layers become damaged or broken, its partner must be discarded or destroyed since it has no further useful purpose in a laminate.
A further limitation encountered with the presently utilized process of free-sagging a pair of glass layers by means of a peripheral mold, lies in the fact that the glass composition of each layer must be substantially identical in order to obtain sagging uniformity in the freely sagged portion. That is, if two layers of glass having different compositions were simultaneously sagged by means of a peripheral mold, the composition having the higher softening point would sag at a much slower rate than the sheet composition having a lower softening point, and accordingly the free-sagging cycle would not produce a matched pair of sheets which could be laminated into a composite laminate.
Accordingly, the present invention has overcome the problems encountered with present day sagging operations by utilizing substantially identical inner and outer shell molds for sagging inner and outer layers of a laminate which may be of various compositions, and which permit the random lamination of such sagged outer layers with such sagged inner layers to form a composite curvilinear glass laminate.